Orthogonal frequency division multiplexing (OFDM) is a multicarrier transmission technique in which a user transmits on many orthogonal frequencies (or subcarriers or tones). The orthogonal subcarriers (or tones) are individually modulated and separated in frequency such that they do not interfere with one another. This provides high spectral efficiency and resistance to multipath effects. An orthogonal frequency division multiple access (OFDMA) system allows some subcarriers to be assigned to different users, rather than to a single user.
The performance of a wireless network may be improved by implementing channel quality feedback. Receiving stations (e.g., subscriber stations) in a wireless network measure selected parameters of the received signal. The measured parameters and, optionally, calculated values derived from the measured parameters are then transmitted back to the wireless network in a standard message, sometimes called a channel quality indicator (CQI) message. The network then uses the CQI information to optimize the signal transmitted in the forward channel (or downlink), thereby improving reception in the subscriber stations. Similar techniques may be used by the subscriber stations to improve performance in the reverse channel (or uplink).
A variety of channel quality feedback techniques are commonly used in multichannel wireless networks, such as OFDM and OFDMA networks. However, multichannel wireless networks typically allocate a subband containing a group of subcarriers (or tones) to each subscriber station and these conventional channel quality feedback techniques commonly transmit an absolute channel quality indicator (CQI) value for each subband. Transmitting an absolute CQI value for each subband requires a prohibitive amount of feedback overhead.
Therefore, there is a need for improved OFDM and OFDMA transmission system that minimize the amount of bandwidth required to provide channel quality feedback.